Show a card box, and borrow a coin that is later signed on both sides. You show the box empty and even your hands are empty. You close the lid. Take the coin and smack it to the bottom. You show your hand completely empty. The coin goes right through the box. And yes, the coin is in the box. Now you do it again. The coin again goes through the box -- you give it to your spectator to open. Let them examine the coin inside, which is his/her signed coin. Everything is examinable.
It starts clean and ends clean. It is the perfect quick trick to amaze your audience!